The 'Tenna Dipper: A "Poor Man's" Antenna Analyzer. Mac A. Cody AE5PH

Transcription

1 The 'Tenna Dipper: A "Poor Man's" Antenna Analyzer Mac A. Cody AE5PH

2 Introduction Who am I? What is antenna tuning? What is antenna analysis? The minimalist approach History of the 'Tenna Dipper My spin on a homebrew 'Tenna Dipper Using the 'Tenna Dipper to tune an antenna Using the 'Tenna Dipper to analyze an antenna Comparison Testing: 'Tenna Dipper vs minivna Pro Questions?

3 Who Am I? Kansas State University: BSEE '79, MSEE '81 (Digital signal processing and instrumentation) Motorola GED, Chandler AZ: '81 - '88 Sunair Electronics, Ft. Lauderdale FL: '88 - '92 (Adaptive Link Establishment modem) '92 - Present: E-Systems/Raytheon IIS, Garland TX Technician and General Class: '09 - KF5DAF Amateur Extra: '10 AE5PH Interests: SDR, home brewing, EMCOMM

4 What Is Antenna Tuning? Adjusting an antenna such that it is near or at resonance for a desired range of frequencies Adjusting an antenna such that it has nearly the same impedance of a connected radio for a desired range of frequencies Adjusting a transmatch to compensate for the impedance mismatch between an antenna and a radio for a desired range of frequencies

5 Using a Radio and SWR Meter Can Be Problematic Many radios and transmatches do not have builtin SWR meters (especially true for home brew radios) Good SWR meters can be expensive and fragile, either electrically, mechanically, or both Using a radio to tune the antenna may damage the radio, the SWR meter, or both An SWR meter is one more piece of radio gear to hook up (and transport, if in the field)

6 What Is Antenna Analysis? Determining the impedance of an antenna over a range of frequencies Determining the Voltage Standing Wave Ratio (VSWR) of an antenna over a range of frequencies Determining the resonant frequency (or frequencies) of an antenna Determining the gain pattern of an antenna

7 Using an Antenna Analyzer Can Be Problematic They can be expensive (good ones are very expensive) - $99 to $$$$ Many antenna analyzers must be calibrated in order to produce reliable, realistic results The operator must use the antenna analyzer properly to obtain reliable, realistic results Improper use of an antenna analyzer can damage it, resulting in an expensive repair bill (or the purchase of a new analyzer, ouch!)

8 The Minimalist Approach Antenna tuning: Determine that an antenna or transmatch are adjusted such that the radio is not likely to be damaged during transmission for a desired range of frequencies Antenna analysis: Determine the range of frequencies that an antenna is unlikely to cause damage to the radio during transmission These similar goals can be accomplished with one, relatively simple and (potentially) inexpensive piece of test equipment: A Resistive SWR Bridge

9 A Resistive SWR Bridge A Wheatstone bridge with the antenna as the unknown resistance Zx (blue circuitry) The green circuitry measures the forward voltage The yellow circuitry measures the reflected voltage The red circuitry measures the bridge imbalance, hence the ratio between Zx and 50Ω Only the red circuitry is required if only an indication of imbalance is needed

10 History Of The 'Tenna Dipper Created by Steve "Melt Solder" Weber, KD1JV A digital VCO (74HC46) drives an antenna resistance bridge (Harmonics? You betcha!) Four frequency bands are set by four trimmer pots forming a tapped voltage ladder, with another trimmer pot used for fine adjustments Two 2N3904 transistors, arranged as a Darlington pair, drive an LED to indicate the amount of antenna mismatch Provides a tap for your frequency meter Fits in an Altoids tin with a 9V battery!

11

12

13

14 4SQRP 'Tenna Dipper Kit Commercialized version of the 'Tenna Dipper formerly sold by the Four State QRP Group Added a modified version of the Arizona ScQRPions Stinger frequency counter chip, which reports the frequency measured in Morse code at 27 WPM Isolates VCO and frequency counter from bridge via a logic gate Still fits in an Altoids tin (if you trim the PCB) with a 9V battery

15

16

17 Hendricks Deluxe 'Tenna Dipper Kit Currently sold as a kit by Hendricks QRP Kits for $75 (Is there value for this cost?) Provides frequency counter with a four-character LED, with two frequency scales Two MPS5179 RF transistors to improve detector performance at higher frequencies One, single turn pot to set frequency in two bands Kit includes components, PCB, decals, and an unfinished case Yahoo group provides support

18

19

20

21 My Spin On a Homebrew 'Tenna Dipper PIC micro-based LED frequency counter designed by Wolfgang "Wolf" Büscher, DL4YHF Diode separates bridge from VCO and frequency counter to avoid signal reflection interference Still uses two MPS5179 RF transistors, after some enlightening experimentation Ten-turn pot to improve frequency selectivity Created single-sided PCB using Eagle CAD Discovered frequency sensitivity issues - VCO, transistors, bridge transformer? Homebrewing is a learning experience!

22

23 AE5PH 'Tenna Dipper PCB

24 Completed AE5PH 'Tenna Dipper

25 Power Output Frequency Sensitivity Power Output Over Frequecy Range (50Ω Load) Power (W) 0.02 Power Power Power Power 0.02 w/ Diode (W) w/o Diode (W) w/ HC04 (W) w/ AC04 (W) Possible causes of power loss are VCO duty cycle reduction, transistor gain frequency roll-off, or frequency sensitivity in the bridge transformer Hendricks QRP Deluxe 'Tenna Dipper probably has this problem too! Frequency (MHz)

26 Using the 'Tenna Dipper to Tune an Antenna Select the frequency band and adjust the pot to display the desired frequency Adjust the antenna until the LED brightness "dips" to a minimum, or... Adjust the transmatch until the LED brightness "dips" to a minimum Adjust the pot and watch the brightness of the LED and the frequency counter value to estimate the match frequency range Multiple LED "dips" may be due to VCO harmonics

27 Using the 'Tenna Dipper to Analyze an Antenna Switch between bands and adjust the pot until the LED brightness "dips" to a minimum The frequency counter displays a local maximum resonance point Adjust the pot and watch the LED brightness and the frequency counter to get a sense of the range of locally maximal resonance Perform this task over the frequencies of interest, as there may be multiple resonance points Multiple LED "dips" may be due to VCO harmonics

28 Comparison Testing How does the 'Tenna Dipper compare to a real antenna analyzer? The mini Radio Solutions minivna Pro is a PCbased Vector Network Analyzer (VNA) It will be used to provide truth measurements for a test antenna Provided by Doug Knabe KN5DK

29 Testing at QTH of Doug KN5DK

30 Subject: Homebrew Buddipole A Buddipole is a walking portable, loaded dipole developed by Budd Drummond W3FF Band selection (20, 17, 15, 12, or 10 meters) through selection of loading coils Tuning done by adjusting the length of the whips

31 Buddipole Setup Buddipole configured with 20-meter coils Black whip - Fully extend all five sections Red whip - Fully extend the inner four sections and extend the outer, fifth section by 3½ inches Raise Buddipole to sixteen feet above the ground Goal: Operate at MHz with an SWR < 2:1

32 Reflection Measurement

33 Single Frequency Measurement

34 Smith Chart Measurement MHz

35 'Tenna Dipper Results LED dimmed, but did not fully extinguish, at about MHz This is the frequency at which the complex impedance vector is closest to 50 Ohms Errors in the 'Tenna Dipper's frequency counter are not taken into account Bench testing indicated that frequency counter may be have about ±10 KHz error

36 'Tenna Dipper Tuning Table Red Aerial Whip Sections* Frequency where LED is Dimmest 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz 4 Sections of Section MHz *Black aerial always has all five sections of whip fully extended

37 Questions? Thanks and 73, Mac / AE5PH...and a big thanks to Doug Knabe KN5DK for his hospitality and support during testing!

38 Back-up Slides

39 Smith Chart Measurement MHz